X-ray imaging apparatus and method using a flat amorphous silicon imaging panel

ABSTRACT

An x-ray imaging apparatus receives an image-carrying x-ray beam on a flat amorphous silicon imaging panel with a light detector unit disposed behind. The imaging panel is of a multi-layered structure having sequentially a light-blocking layer which is opaque to visible light but transmissive to x-rays, a converting layer of a phosphorescent material for converting x-rays incident thereon into visible light, and a two-dimensional array of photosensitive elements of an amorphous semiconductor material such as amorphous silicon, adapted to undergo a detectable change in electrical characteristic in response to impingement of light. The light detector unit may be a simple light detector for receiving the light emitted from the converting layer and passed through regions between neighboring pairs of the array of photosensitive elements. Since the energy of light thus detected is directly proportional to the total light energy emitted from the converting layer, the output signal from such a light detector unit can be conveniently used for the exposure control of the imaging panel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This ia a continuation application of U.S. application Ser. No.08/684,646 filed Jul. 19, 1996 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an x-ray imaging apparatus and method using aflat imaging panel x-ray detector, and, more particularly, for such apanel imager having an array of light-sensitive elements comprising anamorphous semiconductor material such as amorphous silicon.

In view of many disadvantages associated with x-ray image intensifiersand film of conventional types such as large bulk, complexity andincorporation of moving parts, U.S. Pat. No. 4,672,454 disclosed a flatamorphous silicon imaging panel, comprising an array of light-sensitiveelements, as small as about 90 microns to a side and formed from andeposited semiconductor material such as amorphous silicon. For usingsuch an x-ray imager effectively, however, appropriate means arerequired for providing signals representing the x-ray dose beingreceived. For single shot fluorography (wherein snapshots are taken withan electronic device), a signal representing total integrated dose willbe required. For fluoroscopy (wherein an electronic device is used forcontinuous imaging), on the other hand, a signal representing theinstantaneous x-ray flux will be needed. Although various apparatus forexposure control have been available for radiography (or direct imagingon a film) and fluoroscopy with x-ray intensifiers and televisioncameras, there have not been any suitable exposure control apparatus ormethod for use with a flat amorphous silicon imaging panel.

U.S. Pat. No. 3,995,161, for example, disclosed an x-ray exposure deviceusing a multiple-section ion chamber with integrating capacitors toprovide measures of dose in several areas of a film, but ion chambersare incompatible with amorphous silicon panels because they are toobulky and require high voltages which are likely to interfere with thepanel operation. Moreover, they require power supplies which producenoise that is likely to harm the signal quality from the panel. U.S.Pat. No. 4,517,594 disclosed an x-ray installation whereby a smallpercentage of light outputted from an x-ray image intensifier isre-imaged on a segmented photodetector, but there is no x-rayintensifier with a flat panel, nor is there any means for re-imaging.U.S. Pat. No. 4,171,484 disclosed a direct view fluoroscopic imagingsystem with an image intensifier tube and a high-voltage bias supplytherefor. Dose signals are derived from the variations in the outputfrom a phosphor display screen. This scheme, however, admits noselection of image sampling area, and amorphous silicon panels have noequivalent power supply means. U.S. Pat. No. 4,679,217 disclosed a filmcassette with small scintillating screens for producing light to bedetected by photodetectors in the cassette holder. This scheme requiresauxiliary equipment to produce electrical signals for use by a generatorand therefore prevents utilization of the cassette exposure control inany film holder except those designed with the auxiliary electronicsincluded. Moreover, the light output of the screens, if used withamorphous silicon panels, does not truly represent the panel exposure asit would when used with a film. U.S. Pat. No. 4,442,537 disclosed asystem which uses a television camera to measure the output from anx-ray image intensifier. The output from the television camera tube isused to generate a regulating signal for the control unit. If such avideo signal is generated from an amorphous panel, it will not beproduced until scanning occurs, and it will be too late to control thex-ray dose for fluorographic use.

As illustrated by this limited number of examples given above, prior artexposure control apparatus cannot fulfil all requirements for size,power consumption and compatibility with the characteristics ofamorphous silicon panels.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a convenientx-ray imaging apparatus and method for use with a flat imaging panelhaving an array of light-sensitive elements comprising an amorphoussemiconductor material such as amorphous silicon.

An x-ray imaging apparatus embodying this invention, with which theabove and other objects can be accomplished, may be characterized ascomprising a flat amorphous silicon imaging panel and a light detectorunit. The imaging panel is of a multi-layered structure havingsequentially a light-blocking layer which is opaque to visible light buttransmissive to x-rays, a converting layer of a phosphorescent materialfor converting x-rays incident thereon into visible light, and atwo-dimensional array of photosensitive elements comprising an amorphoussemiconductor material such as amorphous silicon, adapted to undergo adetectable change in electrical characteristic in response toimpingement of light.

The light detector unit is disposed behind this imaging panel, oppositeits energy-incident surface through which an image-carrying beam ofx-rays is projected onto the panel. The light detector unit may be asimple light detector which will receive the light emitted from theconverting layer and passed through regions between neighboring ones ofthe array of photosensitive elements. Since the energy of light thusdetected is directly proportional to the total light energy emitted fromthe converting layer and, thus, received by the photosensitive elements,the output signal from such a light detector unit can be usedconveniently for the exposure control of the imaging panel. The lightdetector unit may alternatively have its own converting layer coveringits light-detecting layer such that the residual x-rays which havepassed through the imaging panel without being absorbed by theconverting layer can be detected. If two or more of such light detectorunits of either or both types are used, detection signals therefrom maybe selectively used by means of a switching means or combined in aspecified proportion by means of a weighting means for the purpose ofexposure control.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a schematic of an x-ray imaging system incorporating anapparatus embodying this invention;

FIG. 2 is a schematic sectional view of a portion of x-ray imagingapparatus embodying this invention;

FIG. 3 is a schematic sectional view of a portion of another x-rayimaging apparatus embodying this invention;

FIG. 4 is a schematic sectional view of a portion of still another x-rayimaging apparatus embodying this invention;

FIG. 5 is a block diagram of an exposure control means associated withan x-ray imaging apparatus; and

FIG. 6 is a block diagram of another exposure control means.

DETAILED DESCRIPTION OF THE INVENTION

An x-ray imaging system incorporating an apparatus which embodies thisinvention is schematically shown in FIG. 1. An x-ray generator tube 10generates a beam of x-rays 12 adapted to pass through an object such asa patient to be x-rayed 14 and be received by a flat amorphous siliconimaging panel 20. As shown in FIG. 2, the panel 20 is of a multi-layeredstructure having an energy-incident surface 22, through which theimage-carrying x-rays 12 are received, and a light-detecting surface 24opposite thereto. Since aforementioned U.S. Pat. No. 4,672,454, hereinincorporated by reference, describes a detector panel of this type indetail, FIG. 2 is intended to show the structure of the panel 20 onlyschematically as comprising a light-blocking layer 26, an x-rayscintillator layer 28 and a two-dimensional array of photosensitiveelements 30 on a glass substrate 32, numeral 31 indicating an insulatinglayer filling the space in between. The light-blocking layer 26 isopaque to visible light but transmissive to x-rays and may comprise athin layer of aluminum. The x-ray scintillating layer 28, which iscontinuous over the light-blocking layer 26, is of a phosphorescentmaterial such as doped cesium iodide or gadolinium oxysulfide adapted tophosphoresce, when impinged upon by the x-rays 12, to convert the x-rayenergy into light energy in a different range. The light-sensitiveelements 30, which are themselves opaque, comprise an amorphoussemiconductor alloy, and preferably amorphous silicon, capable ofundergoing a detectable change in electrical characteristic in responseto light received from the x-ray scintillating layer 28. Although notshown in FIG. 2 (but illustrated and explained in aforementioned U.S.Pat. No. 4,672,454), the panel 20 further includes means forindividually detecting the electrical characteristic of theselight-sensitive elements 30 and outputting signals indicative thereof(as shown schematically in FIG. 1). The light-sensitive elements 30 canbe made to have dimensions of only about 90 microns on a side and henceare capable of representing an x-ray image with a high resolution. Inorder to increase absorption of light from the x-ray scintillating layer28, light-transmissive regions 34 between neighboring pairs of thelight-sensitive elements 30 are minimized but are yet adapted totransmit light from the x-ray scintillating layer 28 therethrough to thelight-detecting surface 24.

For the purpose of automatic control of the x-ray dose from the x-raygenerator tube 10, detecting means of different kinds may be usedaccording to this invention, depending on the energy range expected andthe transmission of the panel 20 to both x-rays and light.

FIG. 2 shows a simple light detector unit 40, which may be a siliconphotodiode or phototransistor, an avalanche photodiode, or a miniaturephoto-multiplier, mounted behind the panel 20 (that is, either on itslight-detecting surface 24 or sufficiently proximally thereto such thatthe separation therefrom will not significantly affect the amount oflight collected from the x-ray scintillating layer 28). As describedabove, there are light-transmissive regions 34 between mutually adjacentones of light-sensitive elements 30 through which visible light emittedfrom the x-ray scintillating layer 28 passes to reach the glasssubstrate 32 and received therethrough by the light detector unit 40.Because the total light energy that is transmitted is in exactproportion of to the amount of light energy received and detected by thelight detector unit 40, the proportionality can be easily calibratedpreliminarily and the x-ray dose from the x-ray generator tube 10 can beeasily determined from the charge read out from the light detector unit40.

The type of detector unit to be used will depend on the amount of lightwhich passes through the panel 20, depending on the panel transparencyand x-ray dose rate. At high levels, a simple photodiode is suitable. Atvery low dose levels, an avalanche photodiode or even a photomultipliermight be needed. Because all of these devices are small compared to thesample area typically desired, some sort of shaped light collectingdevice ("a large area collector") may be needed. Examples of such acollecting device include a sheet of acrylic plastic (or polymethylmethacrylate).

Another type of detector unit 42 which may be used and illustrated inFIG. 3 is characterized as being adapted to detect x-rays rather thanvisible light, comprising a scintillator 45 in front of a light detector46. The x-ray beam 12, incident onto the panel 20, is not totallyabsorbed by the x-ray scintillating layer 28 to be converted into lightenergy. A certain portion of the incident x-rays penetrates the panel 20and reappears on the opposite side. The scintillator 45 is adapted tocapture such left-over x-rays and the light energy thereby generated isreceived by the photosensor 46, which outputs a detection signal (notshown) indicative of the energy detected thereby. A detector unit ofthis type is less accurate because it measures the residual x-rays,rather than the absorbed x-rays directly, but this type of detector unitmay be necessary where the transparency of the panel 20 is too low topermit direct detection of the light or where the scintillator does notproduce sufficient light for proper operation of the detector. Given thevoltage of the x-ray generator tube 10 and the knowledge of particulartechnique being used and the absorption characteristics of the panel 20,however, it is possible to obtain some approximate calibration, relatingthe detection signal with the x-ray dose. It is also to be noted thatamorphous silicon panels are much more forgiving of exposure errors thanfilms are. Because the residual x-rays passing through the panel 20 aregenerally still well collimated, unlike the phosphorescent light emittedfrom the x-ray scintillating layer 28, detector units of this kind formeasuring left-over x-rays need not be mounted directly on the panel 20,as illustrated in FIG. 3.

Where the x-ray energy is relatively high (say, over 150 kev), it may bedesirable to remove the light detector unit 40 from the x-ray beam toavoid long-term damage to the detector unit. In such a case, a bundle 48of elongated non-coherent plastic fiber-optic material may be used, asshown in FIG. 4, to transmit the light from its light-receiving surfaceto the opposite end connected to the light detector unit 40. A radiationshield (not shown) may be provided, whenever necessary.

Although not separately illustrated, a fiber-optic bundles may be usedfor bringing together samples from remote areas on the light-detectingsurface 24 to a single light detector unit, or a fiber-optic bundle maybe used in connection with a large area collector.

In situations where it is desired to monitor multiple portions of thepanel 20, say, for adapting to different anatomical examinations, aplurality of detector units may be provided, although not separatelyillustrated. Each of the plurality of such detector units may be adapted(being of the type shown in FIG. 2) to detect light from the x-rayscintillating layer 28 or (being of the type shown in FIG. 3) to detectlight from its own scintillator, or the left-over x-rays which havepenetrated and passed the panel 20. In each of such applications,fiber-optic bundles may be used as shown in FIG. 4.

FIG. 5 shows an example of exposure control means 50 for controlling thex-ray generating tube 10 (shown in FIG. 1) according to the outputs fromthe light detector units 40 or 42, especially where a plurality of suchdetector units are used in the system. Output signals from the lightdetector units 40 or 42 are received by a selector switch 52, of whichthe function is to select one of the plurality of detector units inresponse to selector control signals 53 inputted by the user and toallow only the detection signal from the selected detector unit to passtherethrough. The detection signal which has been allowed to passthrough the selector switch 52 is amplified by a signal amplifier 54.The amplified detection signal serves directly as the instantaneous doseoutput to control the x-ray dose from the x-ray generator tube 10 foronline fluoroscopy. A portion of the amplified detection signal may bereceived by a signal integrator 56 to calculate the total doserepresented by the received during a specified time interval between astart time and a stop time inputted through a start/stop control signal57. The output from the signal integrator 56 serves as the total doseoutput to control the x-ray generator tube 10 for single-shotfluorography.

Such a control system with a switch suffices where only simpleexaminations are to be performed. Since each detector unit may includesampling of an extended area by use of light collectors and since theremay be overlapping areas, a one-to-one anatomical programming method ispossible. This scheme requires much advance planning, however, so a morecomplex method and scheme may be desirable.

FIG. 6 shows another example of exposure control means 60 which allowsproportional mixing of the various signals as required by the anatomicalconfigurations. Detection signals from the plurality of detector unitsare individually amplified by corresponding ones of signal amplifiers 61and received by a weighting means 62 for proportionally mixing theseindividually amplified detection signals according to the weightingscheme communicated through weighting control signals 63 inputted by theuser. In other regards, the control means 60 of FIG. 6 is the same asshown in FIG. 5 and hence these components that may be identical areindicated by the same numerals and not repetitively described.

With exposure control means as shown in FIG. 6, substantial additionalflexibility can be provided in the control. An additional switch (notshown) may be provided to permit remote selection of the proportional orintegrated signal outputs rather providing these on separate lines.

The invention has been described above with reference to only a limitednumber of examples, but these examples are intended to be merelyillustrative, not as limiting. Many modifications and variations arepossible within the scope of this invention. For example, the directlight detection schemes of this invention can be applied also to anylight-detection applications for panels, not limited to those in whichlight is generated by an x-ray scintillator. In fact, any spatiallydistributed image-carrying form of electromagnetic or acceleratedparticle (such as electron) beam energy within an appropriate energyrange can be used on an appropriate (such as solid state) energyconverting means to generate light in another energy range. Thisinvention has applications to a variety of scientific instruments inwhich optimum performance depends on reception of a sufficient amount ofaccumulated light before read-out. In summary, all such modificationsand variations that may be apparent to a person skilled in the art areintended to be within the scope of this invention.

What is claimed is:
 1. An imaging apparatus comprising an imaging panelwith a multi-layered structure having an energy-incident surface and alight-detecting surface opposite to each other and a plurality ofdetector units with an energy-receiving surface disposed proximally tosaid light-detecting surface, said imaging panel including:alight-blocking layer which is opaque to visible light and transmissiveto an incident form of imageing energy projected on said energy-incidentsurface; a converting layer between said light-blocking layer and saidlight-detecting surface for converting said incident form ofimage-carrying energy into light energy; and an array of photosensitiveelements between said converting layer and said light-detecting surface,said elements undergoing a detectable change in electricalcharacteristic in response to impingement of light; at least one of saiddetector units being adapted to receive directly from said imaging panelthat portion of said incident form of said image-carrying energy notconverted into light energy by said converting layer to provide a firstoutput detection signal indicative of the energy received thereby; andat least another of said detector units being adapted to receivedirectly from said imaging panel said light from said converting layerpassing between mutually neighboring pairs of said photosensitiveelements to provide a second output detection signal indicative of theenergy received thereby.
 2. The imaging apparatus of claim 1 whereinsaid detector unit for detecting said incident form of saidimage-carrying energy comprises a light detector and a scintillator,said scintillator being adapted to convert said portion of said incidentform of said image-carrying energy not converted by said convertinglayer into light energy, said light detector being attached to saidscintillator and adapted to output said detection signal which isindicative of the light energy received thereby from said scintillator.3. The imaging apparatus of claim 1 wherein said first output detectionsignal and said second output detection signal are selectively addressedby a switching means for the purpose of exposure control.
 4. The imagingapparatus of claim 1 wherein said first output detection signal and saidsecond output detection signal are combined in specific portions by useof a weighing means for the purpose of exposure control.
 5. The imagingapparatus of claim 1 further comprising an exposure control means foroutputting instantaneous dose signals indicative of energy beingreceived instantaneously by said imaging panel.
 6. The imaging apparatusof claim 5 wherein said exposure control means include a selecting meansfor selecting one of said plurality of detector units and outputtingsaid instantaneous dose signal from said selected detector unit.
 7. Theimaging apparatus of claim 5 wherein said exposure control means includea weighting means for proportionally mixing the detection signals fromsaid detector units according to a weighting scheme inputted theretothrough weighting control signals.
 8. The imaging apparatus of claim 6wherein said exposure control means further includes an integrator foraccumulating the instantaneous dose signals over a specified period oftime and thereby outputting a total dose signal indicative of the totaldose of energy received by said selected detector unit.
 9. The imagingapparatus of claim 5 wherein said exposure control means furtherincludes an integrator for accumulating the instantaneous dose signalsover a specified period of time and thereby outputting a total dosesignal indicative of the total dose of energy received by a combinationof said detector units according to said weighting system.
 10. Animaging method comprising the steps of:causing a selected form of energyto penetrate and pass through a target object to thereby formimage-carrying energy and projecting said image-carrying energy onto anenergy-incident surface of an imaging panel with a multi-layeredstructure; causing said image-carrying energy to pass through alight-blocking layer of said panel; thereafter converting saidimage-carrying energy into light through a converting layer of saidpanel, said panel having an array of photosensitive elements behind saidconverting layer, said elements being capable of undergoing a detectablechange in electrical characteristic in response to impingement of light;detecting said change to obtain an image of said target object; using afirst detector unit outside a light-detecting surface of said imagingpanel which is opposite said energy-incident surface to detect lightfrom said converting layer passing between mutually neighboring pairs ofsaid photosensitive elements and a second detector unit to detect saidimage-carrying energy of said incident form which has penetrated andpassed through said imaging panel; and outputting a combined detectionsignal indicative of the energy received by said imaging panel.
 11. Themethod of claim 10 wherein said selected form of energy is a beam ofx-rays.
 12. The method of claim 10 wherein said first and seconddetector units are disposed at different positions, said method furthercomprising the steps of:inputting a weighting scheme; proportionallymixing the detection signals from said detector units according to saidweighting scheme to thereby obtain a mixed detection signal; andoutputting an instantaneous dose signal on the basis of said mixeddetection signal.
 13. The method of claim 10 further comprising the stepof accumulating the instantaneous dose signals over a specified periodof time to obtain a total dose signal indicative of the total dose ofenergy received during said period of time by said detector units.